I'm not sure what else you're looking for. He has historical and current valid sources of income. Whilst we don't have a signed statement from him on his accounts it's not exactly heresay.
The abridged version - He started in the city, from which he started buying property, he was hot on statistical analysis, got into betting companies, went to Asia and became one of the world's best experts on Asian long odds betting. Got picked up by Victor Chandler and ran their division on it...